UMTS Radio Access Network uses ATM backbone (for carrying both
signaling and user data). So let us have quick brush-up on ATM.

In typical (switched) digital network, multiple nodes are
connected not directly but rather through a mesh of switching
nodes. This backbone network is called switching network and it
fills up the gap between network layer and physical layer. The
data would be in the form of chunks (packets) and the switching
network may provide connection oriented and/or connection less
services depending on switching techniques used. The connection is
more likely to be virtual in nature i.e. may not be mapped
directly to "dedicated channel" in physical layer. In fact, one
the main intentions of having switching network is efficiency.
Consider internet traffic - it is bursty in nature whereas video
and voice call traffic is likely to be low delay/constant bit rate
in nature. Switching network should be able to support all types
of traffic while efficiently using physical resources. Switching
network achieves this by providing generic interface in terms of
(virtual/logical) connections which higher layers can use as per
their requirements. Multiplexing is the key here. Switching
techniques should be able to multiplex (and de-multiplex) various
types of traffic keeping their individual requirements in mind.

Frame relay is one such switching technique wherein data is
transmitted in terms of variable size frames.

ATM is also a packet switching technique, but uses fixed size
packets called cell; ATM is also known as cell switching
technique. The cells are of size 53 bytes - 5 bytes for header and
48 for data. It uses statistical TDM operation i.e. instead of
allocating time slots (cells) to a connection dedicatedly, cells
are sent only when there is data to be sent. This way bandwidth
can be allocated dynamically and with fine granularity.

As ATM network divides higher layer data in terms of cells with
each cell having header, overhead is more compared to variable
packet switching techniques. This looks interesting: ATM is still
recommended for higher speeds. ATM is preferable when we want to
high speed networks (like Broadband ISDN) based on high speed
physical medias like optical fiber. The reason is: most of ATM
cell switching is possible to be done in hardware due to fixed
size of cells that is to be switched. This is not possible in
other packet switching techniques where due to variable size
nature of packets, software intervention is needed to and/or
costlier hardwares are needed for switching.

Connections are managed with the help of virtual paths and virtual
connections. Paths are configured, established, and released by
network. A path consists of multiple connections. Connections can
be made by user or network. Header field of cell contain
identifier for virtual path (VPI) and virtual connection (VCI).
VPI + VCI identify virtual channel. Switching nodes makes sure
that cells are switched appropriately (i.e. transmitted to next
switching node or user) with the help of mapping between incoming
and outgoing paths and connections. Connections can be permanent,
semi-permanent, or switched. Permanent connections are not
established, but rather they are configured during subscription
time. Switched connections require establishment procedure.
Preassigned VCI values are used for basic/mandatory functions
related to signaling and control.

Note that ATM do not distinguish between higher layer signaling
traffic and user traffic. Both types of data are carried in cells
including ATM's own signaling and control traffic.

ATM has three layers:

ATM Adaptation layer (AAL)

ATM layer

Phy layer

AAL has different types of interfaces from Type 1 thru 5,
distinguished based on type of service parameters (like connection
oriented/less, constant or variable bit rate etc.). AAL is
responsible for converting higher layer data to cell format to
transmit over ATM and other way round (segmentation and
reassembly).

Physical layer does the actual transmission. ATM does not specify
particular media for transmission, but high speed and high
bandwidth medias (higher than T3/E3 speeds of 44.74 Mbps) are best
suited. Normally optical fibers are preferred. Optical fibers have
high bandwidth, no magnetic interference, no crosstalk between
fibers, low attenuation (unlike wired medias). In North America,
it is popularly known as SONET (Synchronous Optical NETwork) and
internationally SDH (Synchronous Digital Hierarchy). Physical
transmission is divided in 51.84 Mbps blocks, called OC-1 to
2488.32 Mbps (OC-48). OC-n is n times OC-1, this makes
multiplexing and de-multiplexing easier.